Role of Pressure in High Velocity Nasal Insufflation
Thomas L Miller, PhD
This information is provided in response to requests from users to elaborate on the role of pressure in a high flow nasal cannula system. This educational paper provides summary material related to flow and pressure from the respiratory and neonatal literature. Material contained herein is not designed to provide clinical practice guidelines and is consistent with official instructions for use for Vapotherm.
Clinical Indications for Use for Vapotherm® Technology
Vapotherm, Inc. manufactures high flow humidification devices and patient circuits for use in respiratory support for neonatal, pediatric and adult patients. These products are not intended for use as continuous positive airway pressure (CPAP) devices, but rather as high flow systems to deliver conditioned breathing gases. Vapotherm recommends that users always maintain an open system, including applying a cannula that does not occlude more than 50% of the patient’s nares.
Precision Flow® is intended for use to add warm moisture to breathing gases from an external source for administration to a neonate/infant, pediatric and adult patients in the hospital, subacute institutions, and home settings. It adds heat and moisture to a blended medical air/ oxygen mixture and assures the integrity of the precise air/oxygen mixture via an integral oxygen analyzer. The flow rates may be from 1 to 40 liters per minute via nasal cannula.
The Vapotherm systems and cannula interfaces are designed with small-bore tubing to provide high velocity flow that, when used in the intended flow ranges, can purge the extrathoracic dead space of the respiratory tract in the time between breaths. This subset of high flow nasal cannula (HFNC), known as high velocity nasal insufflation (HVNI), can support the efficiency of breathing. Vapotherm Precision Flow with Hi-VNI Technology can deliver High Velocity Nasal Insufflation.
Background On Mechanisms Of Action In High Velocity Nasal Insufflation
Gas conditioning technology introduced by Vapotherm facilitated the expansion of the use of a nasal cannula. No longer restricted to conventional flow limitations (≤6 L/min in adults; ≤ 2 L/min in infants), nasal cannulae are now being used for HVNI. HVNI refers to the use of nasal cannula gas flows that approach or exceed a patient’s inspiratory flow rates such as to: 1) ensure that the patient will inspire the intended gas composition without entrainment of room air, and 2) provide for other physiologic impacts including purging of end-expiratory gas from the nasopharynx during expiration and development of mild distending pressure.
In a foundational paper by Dysart and colleagues, five potential underlying mechanisms of action for HVNI are identified1:
- Washout of the nasopharyngeal dead space
- Reduction in inspiratory resistance associated with gas flow through the nasopharynx
- Improvement in respiratory mechanical parameters associated with gas temperature and state of humidification
- Reduction in metabolic work associated with gas conditioning
- Provision of mild distending pressure
This paper discusses topics related to the fifth identified mechanism, distending pressure, with respect to nasopharyngeal pressure development and including expected pressure ranges and safety. Specifically, the scope of this paper
will define the relationship between pressure in the patient circuit and the nasopharynx, and identify the factors that contribute to inadvertent pressure development.
High Velocity Nasal Insufflation (HVNI): Respiratory gas therapy where the flow from the external gas source is delivered at high flow and high velocity through a small-bore nasal cannula for the purpose of eliminating entrainment of room air during inspiration and purging expiratory gas from the dead space during exhalation.
Patient Circuit: Tubing connecting the gas source to the cannula.
Anatomical Reservoir: The extrathoracic dead space being purged during HVNI, composed of the dead space of the nasal, oral and pharyngeal cavities.
Pressure: The distending force created when a gas stream comes in contact with resistance.
Flow: The stream or current of respiratory gas through the device and respiratory systems, typically quantified in liters per minute (L/min).
Resistance: A force that tends to oppose flow, resulting in back pressure.
Dead Space (DS): Region of the conducting airways that does not contribute to gas exchange. Under normal circumstances, the DS inadvertently serves as a reservoir of end-expiratory gas that is re-breathed at the onset of a subsequent inhalation.
Resistor: A specific point or region in the flow path that has been identified as having relatively high resistance, resulting in significant backpressure (i.e. a bottleneck).
Flow And Pressure Fundamentals
High flow nasal cannula is intended to be an open system, with flow delivered to a patient via nasal cannula, where the cannula prongs do not occlude the nares and where the patient’s mouth is not held closed. In this open system, the pressure in each compartment is a function of the resistor(s) that lie in series downstream from that compartment. In this regard, circuit pressures will always be substantially greater than pressure in the nasopharynx. To explain why circuit pressures will always be substantially greater than nasopharyngeal pressure, consider Figure 1.
FIGURE 1. SCHEMATIC OF A HIGH FLOW NASAL CANNULA (HFNC) CIRCUIT / PATIENT INTERFACE
R = resistive element; P = pressure compartment impacted by downstream resistance
Figure 1 demonstrates that there are two principle resistors and thus two pressure compartments in the circuit / patient interface. Resistor #1 (R1) represents the nasal cannula and therefore pressure compartment #1 (P1) represents the patient circuit. Resistor #2 (R2) represents the components resistive to gas exhausting from the patient’s nose (around the cannula) and mouth and therefore pressure compartment #2 (P2) represents pressure generated in the nasopharynx. For each pressure compartment, the established pressure is a result of the total downstream resistance (RT; Equation 1); therefore, P1 is always a function of both R1 and R2, while P2 is only ever a function of R2. Furthermore, because under normal conditions R1 is dramatically greater than R2, we can expect P1 to be much greater than P2.
EQUATION 1: DEFINITION OF TOTAL RESISTANCE FOR SERIES RESISTORS
RT = R1 + R2
Where RT is total resistance and R1 and R2 are individual resistors in series
￼Pressure In The Device Circuit
The aforementioned principles translate to practical application of HVNI in the following manner. Because a nasal cannula offers such a high resistance to flow, any device intended to drive high flow rates through a cannula (defined in respiratory care terms as > 6 L/min) must be designed to contain and function under these normally high operating patient circuit pressures. Circuit pressure in the Vapotherm Precision Flow can typically be in the 3 to 4 PSI range. Any attempt to relieve circuit pressures via a pressure relief valve to protect device components, would naturally result in a reduction of actual flow through the cannula thus lessening the intended patient flow (see Figure 2)2. However, the high circuit pressures do not translate to the patient’s airway because this pressure is a function of the cannula resistance which is upstream to the patient.
FIGURE 2. IMPACT OF A PRESSURE RELIEF VALVE IN A HFNC SYSTEM
This figure is reproduced from data presented in Lampland et al2 using a Fisher and Paykel® system with and without a pressure relief valve (PRV) set to 45 cmH2O. With the pressure relief valve in place, the system does not permit more than 2 L/min to pass through the cannula regardless of the flow entering the humidifier.
￼￼Pressure In The Nasopharynx
Nasopharyngeal pressure (positive airway pressure) is determined by three principle factors3:
- the flow setting,
- the patient’s unique anatomical geometry, and
- the leak out of the nose around the prongs and out of the mouth.
In HVNI, the basic flow setting is meant fundamentally to approach or exceed normal inspiratory flow rates so as to eliminate entrainment of room air. At these relatively moderate flow rates only moderate nasopharyngeal pressure can be expected, and flow rates can be titrated upward to enhance nasopharyngeal washout effects without generating substantial increases in pharyngeal pressure. However, Vapotherm emphasizes that during HVNI pressure is not the principle mechanism of action and caregivers should not utilize excessive flows in an attempt to generate substantial distending pressures.
Anatomical size of the patient, at the nares and internally, are factors in determining distending pressure3,4 because anatomy largely defines the resistance to flow passing through and out of the nasopharynx. However, if flow ranges are determined based on predicted normal inspiratory flow rates, then anatomical features are grossly accounted for as these computations account for a patient’s size. However, the relationship between anatomy and flow resistance is more clinically relevant with infants as opposed to adults, because HVNI in infants may exceed a patient’s normal inspiratory flow by several fold. This difference in relative flow rates per patient population is attributed to the relatively greater extrathoracic dead space volume in infants5, which provides a greater opportunity to effect ventilation but at a supraphysiologic flow rate.
The most critical factor in determining nasopharyngeal pressure development when initiating HVNI is the relationship between internal diameter of the nares and the size of the nasal cannula used3,6,7.
The original report of pressure development with a nasal cannula shows esophageal pressure was not recordable when a very small cannula was used, but mild pressure was produced when a larger cannula was used at the same flow rate6. In a bench model, Kahn and colleagues demonstrate that nasopharyngeal pressure development is predominantly a function of the leak around the prongs3, thus making the selection of which nasal prong size to use an important part of applying the therapy.
Vapotherm recommends selecting nasal prongs that have an outside diameter no more than 50% of the inside diameter of the patient’s nares. With this fitting mild distending pressure will develop, which will support the other mechanisms of action; however, there is still adequate room for leak around the prongs. The leak is necessary to allow for a reasonable amount of flush in the nasal cavity to accomplish the actions of dead space washout.
￼￼Expected Nasopharyngeal Pressure Ranges
In the neonatal community, a significant body of literature has been amassed to describe the resultant airway pressures during the application of HFNC, including Vapotherm’s HVNI. Table 1 reports the findings from these studies, which are consistent in agreement that maximum pressures are typically not different from a CPAP setting of 6 cmH2O. Note that in a number of these studies, the protocols called for a closed mouth and occluded nares in an effort to establish greater pressures.
Pressure in the patient circuit is necessary to drive high flows though a nasal cannula, particularly at high velocity. This circuit pressure is isolated from the patient. Airway pressure during HVNI is dependent on factors including flow rate, patient’s size and the relationship between cannula prong size and the internal diameter of the nares. However, pressure generation has been evaluated in a several papers and shown to be moderate.
TABLE 1. NEONATAL AIRWAY PRESSURE STUDIES USING HIGH FLOW NASAL CANNULA
|Study||Journal||Year||# of Infants||Wt Range (gm)||Flow Range (L/min)||Conclusions of Airway Pressure||Relevant Circumstances|
|Saslow8||J Perinatol||2006||18||580–1990||3-5||Not more that CPAP of 6 cmH2O||Esophageal manometry refer- enced to CPAP 6 cmH2O|
|Pyon9||PAS (abstract)||2008||8||< 2000||6-8||Not more than CPAP of 6 cmH2O||Esophageal manometry refer- enced to CPAP 6 cmH2O|
|Spence10||J Perinatol||2007||14||Up to 5||Intrapharyngeal pressure was 4.8 ± 0.5 cmH2O at 5 L/min||Mouth closed and nasal catheter|
|Wilkinson4||J Perinatol||2008||18||534-1868||2-8||Mean pharyn- geal pressure of 5.3 cmH2O at 5 L/min||Nasal catheter|
|Kubicka11||Pediatrics||2008||27||200-3500||1-5||Highest oral cavity pressure recorded was 4.8 cmH2O||Mouth closed with snug prongs|
|Lampland2||J Pediatr||2009||15||1324 ± 424||1-6||Similar to CPAP of 6 cmH2O||Esophageal manometry refer- enced to CPAP 6 cmH2O|